It sounds like you have a air locked block from the low coolant level. Fill the reservior a little over the full line and run for a while with the cap off. When the air comes out, the reservoir will drop quickly. Look for dried coolant on the bottom of the new water pump. New pumps seem to weep at the seals either from too much or not enough pressure. Make sure that the upper hose is not soft, as it can collapse and cause problems. The thermostat must be good, and the small hole should be unblocked and at the 12 o'clock position (the hole allows hot water to get up to the thermostats wax pellet and also lets air pass).
